The invention relates to a method for detection of wind power installations by means of a radar installation.
A wind power installation (WPI) converts the kinetic energy of the wind to electrical energy. Installations having a horizontal rotation axis through which the wind passes have been implemented, comprising a large number of different types of concept. They generally consist of a cylindrical steel tower with a hub at the end, from which three aerodynamically shaped rotors originate. The height of the tower, the height of the hub and the rotor length vary depending on the power of the turbine and/or the installation location. A summary of typical dimensions is shown below:
Category1.535Rotor length (m)384562Hub height (m)60-110 80-105100-120Overall height (m)98-148125-150162-182Speed of revolution (rpm)171612Blade tip speed (m/s)687679
The maximum speed of the rotors is measured at the rotor tips, and is calculated using the formula:u=λ*νwhereu: speed at the rotor tipλ: tip speed ratioν: wind speedThe tip speed ration λ is dependent on the type of wind power installation and is 7 to 8 for the installations which are normally used nowadays. Modern wind power installations operate only in the range between a minimum and a maximum wind speed. Furthermore, they are turned to the optimum position with respect to the wind direction by means of actuating motors, and their rotation speeds are regulated, i.e., the speed of revolution is kept approximately constant, and is varied only continuously.
Fundamentally, the following typical circumstances are known for wind power installations:
a) Fixed position—A wind power installation is always located in the same range cell (x, y), i.e., the range and the azimuth angle are constant.
b) Rotation speed regulation of the rotors—Modern wind power installations vary the rotation speed of the rotors only continuously, i.e., there are no sudden changes in the rotor speed, and therefore in the Doppler frequency shift.
c) Constant operating range—Modern wind power installations require a minimum wind speed for operation, and are switched off above a maximum wind speed. This means that the rotor speed, which depends on the wind speed, varies within a defined range.
d) Physical constraints—The minimum distance between individual installations is predetermined both technically and by physical effects. Phenomena such as wind shadow effect and measures to minimize bird strike, noise production, etc., result in a certain minimum distance between individual wind power installations.
Furthermore, wind power installations are distinguished by a large radar cross section (RCS). The radar cross section in S band (2.7-2.9 GHz) and L band (approximately 1.3 GHz) from wind power installation turbines that are currently in use may be up to 300,000 m2. Considered individually, the rotor blades may achieve a peak value of up to 30,000 m2. In comparison to this, a multi-jet commercial aircraft has an RCS of 5 to 20 m2. The RCS values mentioned above for wind power installations refer to peak values, which are dependent on the position of the rotor blades and on the aspect angle of the radar. The RCS of the wind power installation therefore varies continually and suddenly. [C. A. Jackson, Windfarm Characteristics and their Effect on Radar Systems, Proc. International Conference on Radar Systems, Edinburgh, October 2007]
The continuing and sudden change in the RCS of a wind power installation results in the so-called “glint” effect. This means that echo signals appear completely randomly. When there are a plurality of wind power installations in a wind farm, this leads to mutually independent, random glints in this area. These randomly occurring echoes are in the same range as the Doppler shift in which a large number of possible targets for the radar are also located. This “glint” leads to a series of problems:
a) Clutter: Increased number of detections in the area of windfarms, caused by the echoes from the wind power installation rotors.
b) Desensitization: Reduced probability of detection of an airborne target above or in the vicinity of windfarms, both in the azimuth and in the range direction.
c) Trajectory loss: Tracking of a target in the area of windfarms is impossible.
Exemplary embodiments of the present invention provide a method by means of which a wind power installation can be reliably detected, while avoiding the disadvantages from the prior art.
The method according to the invention comprises the following method steps:
a) a number N of predeterminable sequences Bx, where x=1, . . . , N, of K modulated transmission pulses are transmitted, in each case at a predeterminable pulse repetition frequency, successively in time,
b) a predeterminable number of pulse repetition frequencies are used,
c) after each transmitted sequence, cyclic switching takes place to a different pulse repetition frequency,
d) transmission pulses reflected by an object are received and processed by the radar installation such that received pulses which correspond to the transmission pulses are created, wherein a number R of received pulses are received for each transmission pulse,
e) the range to the object is determined by means of signal delay-time measurement from the received pulses which belong to the sequences Bx of modulated transmission pulses, and a range Doppler matrix is produced, with R rows and N*K columns,
f) a number M of measurement processes are carried out cyclically in an azimuth direction in an azimuth range, wherein N sequences Bx, of modulated transmission pulses are transmitted in the respective azimuth direction for each measurement process, and a range Doppler matrix is produced for each measurement process M,
g) sample values of Z, where Z=2 . . . N, sequences BN of modulated transmission pulses from M measurement processes are arranged in sequence by means of a predeterminable correction term, and are processed, wherein a harmonic oscillation with the least square error with respect to the sample values is calculated by means of a maximum likelihood estimator,
h) a result matrix with R rows and N/Z columns is obtained from the range Doppler matrix with R rows and N*K columns by use of the maximum likelihood estimator, wherein the signal strength, phase and Doppler frequency of the object are known for each cell,
i) after method steps a)-h), two result matrices E1, E2 are produced successively in time and are compared with one another, wherein a range Doppler cell is marked as a location of a wind power installation if
i. the signal strength of a range Doppler cell is above a predeterminable threshold value in both result matrices E1, E2,
ii. the Doppler frequency shift in a range Doppler cell is within a normal value range for wind power installations in both result matrices E1, E2, and
iii. the difference between the Doppler frequencies in a range Doppler cell in both result matrices E1, E2 is below a predetermined threshold value.